Electrical interference protection arrangement for embedded control computers

ABSTRACT

The present invention relates to an arrangement and methods for protection against electrical interference in a control computer ( 1 ) adapted to control at least one external unit ( 2 ). The invention in characterized in that the control computer ( 1 ) comprises hardware modules ( 4 ) designed to personal computer standards. In addition at least one of the modules ( 4 ) is operatively connected to means ( 8 ) adapted to prevent at least the interference which is harmful to the modules from reaching the latter and vice-versa. The interference-preventing means ( 8 ) are further operatively connected to the external unit ( 2 ). The arrangement comprises means adapted to shield the modules and the connection between the modules and the interference-preventing means in order to prevent capacitive/inductive transmission of the interference. The means of shielding comprise an outer casing ( 5 ), essentially enclosing at least the modules and the connection between the modules and the interference-preventing means and shielding them against electromagnetic radiation, together with an inner casing ( 9 ) essentially enclosing at least one of the modules and shielding it against electromagnetic radiation.

FIELD OF THE INVENTION

The present invention relates to an arrangement for protection againstelectrical interference in a control computer. The present inventionalso relates to a method for protecting hardware modules forming part ofa control computer against electrical interference and a method ofprotection against electrical interference emitted by a control computercomprising hardware modules.

BACKGROUND OF THE INVENTION

Programmable industrial control systems have traditionally been made upfrom highly specialized hardware and software modules. In recent years,however, there has been increasing interest in also using the so-calledpersonal computer or PC concept for industrial control and automation.This is due to the fact that the personal computer concept is associatedwith a number of advantages, such as modularity, flexibility,incomparable access to both software tools and hardware modules and highquality, together with well-developed standardization. Personalcomputers, however, are constructed on the basis of the requirements ofan office environment and are therefore in many respects unsuitable foruse in more exacting environmental conditions.

Firstly, contacts and other mechanical parts are often designed for anoffice environment and are not of sufficient mechanical and electricalquality for an industrial environment. Secondly, the mechanical andelectrical design of hardware modules of personal computers is such thatmodules cannot be replaced during operation. This facility is extremelydesirable in the case of industrial control in order to avoid having toshut down an entire production line in the event of a failure in amodule. Furthermore the electrical design of hardware modules is oftenfar too deficient for industrial use. The great majority of modules onthe market lack basic electrical protection, which is always required inan industrial context in order to prevent the destructive orinconvenient effect of incident electromagnetic interference, whetherline-conducted or transmitted through capacitance/inductance. Even ifspecially manufactured hardware modules are used with completeprotection, the problem remains that interference is still given off inthe vicinity of modules that do not have corresponding protection, whichin turn leads to a risk of capacitive/inductive transmission of theinterference. Furthermore the main components according to the PCconcept are basically not designed to withstand the levels ofinterference that an industrial control system i s expected towithstand. By main components we mean here the CPU and the so-called PCbus for electrical communication between the CPU and the constituenthardware modules. The said hardware modules must therefore be protectedfrom the risk of line-conducted or capacitive/inductive interference.

An hermetically sealed construction for use in difficult environmentalconditions, especially in military environments, is described in U.S.Pat. No. 5,570,270. The construction comprises a personal computer and acasing containing printed circuit cards and is intended to protect theprinted circuit cards from electromagnetic interference (EMI) andmechanical influences in the form of high accelerations.

SUMMARY OF THE INVENTION

The present invention is intended to produce an improved arrangement andmethods compared to the prior art in a control computer for use, forexample, in an industrial or medical environment, which provide a goodcompromise between good protection against interference on the one bandand flexibility and interchangeability on the other.

This is made possible by an arrangement of the type stated in theintroductory part, which is characterised in that the control computercomprises hardware modules designed to personal computer standards, inthat at least one of the modules is connected by way of first leads tomeans adapted to prevent at least the line-conducted interference, whichis harmful to the modules, from reaching the latter, in that theinterference-preventing means are connected by way of second leads tothe external unit and that the arrangement comprises means adapted toshield the modules and the connection between the modules and theinterference-prevent means in order to prevent capacitive/inductivetransmission of the interference, the said means of shielding comprisingan outer casing, essentially enclosing at least the modules and theconnection between the modules and the interference-preventing means andshielding them against electromagnetic radiation, together with an innercasing essentially enclosing at least one of the modules and shieldingit against electromagnetic radiation.

This has also been made possible by means of methods of the type statedin the introductory part, which are characterised in that at least thesignals from the external unit to each module are made to pass throughinterference-preventing means, which prevent the signal interferencethat is harmful to the modules from reaching the latter, that thecontrol computer is essentially enclosed by an outer casing, shieldingit against interference in the form of electromagnetic radiation, andthat the hardware modules are essentially enclosed by an inner casingshielding them against the said electromagnetic interference, in whichthe parts of the control computer that are located inside the innercasing are designed in accordance with a personal computer standard andthe parts of the control computer that are located outside the innercasing are designed in accordance with a standard for use in someindustrial environment, for example DIN. The casings are adapted both toprotect the modules from external electromagnetic radiation and toprotect the surroundings from electromagnetic radiation that is emittedby the modules. In addition the signals from each module to the externalunit can also be made to pass through the interference-preventing means,which prevent signal interference in excess of a certain energy levelfrom passing.

The present invention has a number of improvements on the prior art.Among other things, shielding against electromagnetic radiation isproduced in two stages, which function even in environments verysusceptible to interference, for example hospital environments, whereinterference from and by equipment located in the vicinity is entirelyunacceptable. In addition high energies are prevented from reaching theunshielded parts of the control computer by way of the leads.Furthermore the method and the arrangement according to the inventionare, for example, suitable for use in the control and automation ofvarious processes in industry. Valuable advantages in this and othercontexts include the fact that the arrangement according to theinvention can be manufactured at relatively low cost, due among otherthings to the use of standard components and the simplicity of thearrangement, and to the fact that it is possible to replace componentsduring operation.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be described in more detail below in relationto examples of the embodiment and with reference to the attachedfigures, in which:

FIG. 1 shows a control computer and a robot, which the control computeris adapted to control.

FIG. 2 shows an example of an arrangement in the control computeraccording to the present invention viewed from above.

FIG. 3 shows a front view of the arrangement in FIG. 1.

FIG. 4 shows a side view of the arrangement in FIG. 1.

FIG. 5 shows an example of various energy levels in an arrangementaccording to the invention.

FIG. 6 shows an example of a signal adjusting circuit.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

In FIG. 1 a control computer is indicated by reference number 1, thecontrol computer 1 being adapted to control an external unit 2 in theform, for example, of a robot. Of course, it is also possible toconceive of an embodiment with control of a plurality of external units(not shown). In addition the control computer is located in a controlcabinet 16. Leads 3 are run between the control computer 1 and the robot2 for transmitting signals between these units. As will be discussed inmore detail later, the control computer comprises an internal unit,which contains personal computer components, that is components based onstandards according to IBM PC concept and/or pc-104 standard or thelike.

The arrangement shown in FIGS. 2, 3 and 4 is adapted to protect thecomponents forming part of the internal unit against electricalinterference, which can damage or be destructive of the said components,and in the same way to eliminate, or at least considerably reduceinterference emitted from the components in the internal unit. Thecomponents referred to in this description are the hardware modules ofwhich a personal computer is made up, such as the computer mother board,possibly comprising the CPU, memory cards, graphics cards and interfacesto external units in the form of sensors and actuators. In FIG. 2reference number 4 denotes one of the plug-in cards inserted into theinternal unit, in the form, for example, of an IBM-compatible plug-incard for the ISA bus. The space for the internal unit is denoted in FIG.2 by the reference number 12.

The plug-in card inserted in the space 12 is operatively connected tothe external unit 2. The leads 3 from the external unit 2 lead to acasing 5 enclosing a control computer 1 by way of mechanically robustconnection contacts 6, which are arranged to receive signals with highenergy levels and which are shown in FIG. 3. The contacts 6 are designedin accordance with any industrial standard for contacts formulated forthe operating environment contemplated, for example DIN 41652, IEC 807,MIL-C-5015. In one example the casing 5 is designed in accordance withDIN 41494 and IEC 297 for enclosures of standard width (19 inches ˜0.48m) and in the most advantageous case has a height of four height units(1 height unit ˜40.4 mm). The contacts 6 are in turn connected tosignal-adjusting circuits 8, which are adapted to prevent signals withamplitudes or energy levels that interfere with or damage the hardwaremodules of the internal unit or circuits arranged thereon from reachingthe latter and in the same way to prevent the emission of line-conductedinterference. The signal adjusting circuits may be of conventional typeand contain an amplitude limiter and possibly also a frequency limiter(band-pass filter). The said signal-adjusting circuits 8 are alsodesigned in accordance with any industrial standard formulated for theoperating environment contemplated (for example, DIN 41494+IEC 297).From the signal-adjusting circuits 8 the signals are relayed to leads 18by way of contacts 20 designed according to any industrial standard forcontacts, for example DIN 41612. In one embodiment the leads 18 are runto the hardware modules by way of connections 19 conforming to personalcomputer standards.

The entire arrangement with the hardware modules 4 of the internal unit,the signal-adjusting circuits 8 and the leads 18 is enclosed in thecasing 5, which is shielded from electromagnetic radiation and is thusadapted to function as a Faraday cage. Since components conforming topersonal computer standards are more susceptible to electricalinterference than those conforming to industrial standards, the hardwaremodules 4 should be additionally protected against interference. Forthis reason these modules are also enclosed in an inner casing 9shielding them from electromagnetic radiation, this casing also beingadapted to function as a Faraday cage. As can be seen from FIG. 2, theinner casing 9, like the outer casing 5, has ventilation openings 10 and21 respectively. Ventilation openings are preferably located both in thebase and the top of the casings 5, 9 in order to dissipate heatgenerated by the hardware modules. Through the ventilation holes 10, 21,designed so as to maximise the heat radiation, air is allowed to flowthrough the arrangement so that heat generated in the hardware modulesis removed from the arrangement by natural convection. This avoids orconsiderably reduces the need for fan arrangements and filters whichotherwise often create problems in industrial environments.

The shielding construction described above produces a space 11 insidethe outer casing 5 that is largely shielded from radiation. In additionthe inner casing 9 provides shielding from the radiation from thecomponents contained in the space 12, so that very little, if any of thesaid radiation reaches the space 11. The casing 5 therefore ensures thatthe signal-adjusting circuits 8 are not exposed to large doses ofelectromagnetic radiation. In addition the leads 18, the signals ofwhich have been adjusted for the energy levels that the hardware modulescan withstand, are to a large extent protected against externalradiation and against radiation from the components in the space 12. Theinner casing 9, as stated previously, provides both an additionalprotection against external radiation for the components contained inthe space 12 and a protection for the components in the space 11 againstradiation from the components in the space 12. In most industrialenvironments the energy level in the space 12 will be less than that inthe space 11. If, however, the control computer is used for any medicalapplication and is located in a hospital environment with stringentemitted radiation requirements, the energy level in the space 12 will behigher than that in the space 11. Thus the design with two levels ofshielding provides protection both against line-conducted interferencereaching the susceptible modules 4 and vice versa, and againstinterference transmitted through capacitance/inductance reaching thesaid modules and vice-versa.

The signal-adjusting circuits 8 are arranged adjacent to the outercasing 5, accessible for replacement in the even of a fault in the saidcircuits, for example by being able to remove one side 7 of the outercasing. Faults can occur, for example, if signal energies reach thecircuits, the energy levels of which the circuits are not adapted towithstand. The signal-adjusting circuits are of modular construction andin the event of a fault in any of the said circuits can be replaced. Inorder to facilitate fault localisation, indicators 13, in the form oflight-emitting diodes, for example, can be connected to eachsignal-adjusting circuit in order to indicate whether current is flowingthrough this or not. In an advantageous embodiment the signal-adjustingcircuits 8 are designed so that they can be replaced during operation.The electrical characteristics of the circuits 8 are such that removalof a circuit 8 for changing does not result in any signal levels harmfulto the associated module reaching that module. FIG. 6 shows an exampleof a signal-adjusting circuit 8, an indicator 13. which shows the signallevel in the circuit, a part 7 of the casing 5 and a contact connection6.

There are also, as can be seen from FIG. 4, spaces for a power supplyunit 14, mass storage unit 17 and user interface 22 a, 22 b and otheremitters of electromagnetic radiation in the space 11. The power supplyunit 14 is there to supply voltage to the signal-adjusting circuits, thehardware modules and other units that require a power supply. The powersupply unit is fed with alternating current voltage or direct currentvoltage via a connection 15 provided for the purpose and shown in FIG.3. The power supply unit preferably also contains some form of amplitudeand frequency limiter for the reduction of interference irradiated andemitted via leads.

A method of protecting hardware modules forming part of a controlcomputer against electrical interference comprises the following stages.At least one of the modules is operatively connected to an external unitso that electrical signals can be transmitted between each module andthe external unit. Signals from the external unit to each module andvice-versa are made to pass through interference-preventing means. Thesaid means prevent signal interference that is harmful to the modulesfrom reaching the latter and vice-versa in the opposite direction. Thecontrol computer is essentially enclosed by an outer casing shielding itagainst interference in the form of electromagnetic radiation. Thehardware modules are essentially enclosed by an inner casing shieldingthem from the said electromagnetic interference. The parts of thecontrol computer that are located inside the inner casing are designedin accordance with a personal computer standard and the parts of thecontrol computer that are located outside the inner casing are designedin accordance with a standard for use in some industrial environment,for example DIN. The casings protect both the modules from externalelectromagnetic radiation and the surroundings of the control computerfrom electromagnetic radiation that is emitted from the modules.

As will be apparent from the above, the present invention does notrelate solely to protection against radiated energy. In the case ofmedical equipment, for example, it is quite important to preventradiation from the hardware modules of the personal computer fromreaching the surroundings. The arrangement and method according to theinvention also relate to protection against such emitted radiation. Thearrangement and method according to the invention have been shown tomeet EC standards according to medical requirements, for example EN601-1-2.

In one example the arrangement according to the invention is constructedso that four different energy levels are produced, as can be seen fromFIG. 5. The environment in which the control computer was intended to beinstalled outside a control cabinet 16 has an energy level 0. The walls,base and top of the control cabinet may produce energy damping, theenergy level in the control cabinet 16 being lower than that outside thecontrol cabinet; here we call the energy level in the control cabinetenergy level 1. As stated previously, the outer casing 5 providesadditional shielding, the energy level inside the casing being lowerstill; here called energy level 2. The components enclosed by the outercasing 5 are designed, for example, in accordance with a DIN standard.The inner casing 9 creates further shielding, a further energy level,called energy level 3, prevailing inside the inner casing 9. Inside theinner casing the components are designed in accordance with a personalcomputer standard.

What is claimed is:
 1. Arrangement for protection against electricalinterference in a control computer controlling at least one externalunit, wherein the control computer comprises hardware modules designedto personal computer standards, and at least one of the modules isconnected by at least one first lead to at least one means forpreventing at least line-conducted interference which is harmful to themodules from reaching the modules, wherein the means for preventing atleast line-conducted interference includes a signal adjusting circuit;at least one second lead connecting the means for preventing to theexternal unit; and the arrangement further comprises means for shieldingthe modules and the first lead in order to prevent capacitive/inductivetransmission of interference, said means for shielding comprising anouter casing enclosing at least the modules and the first lead forshielding the modules and the first lead against electromagneticradiation, and an inner casing enclosing at least one of the modules andshielding the module against electromagnetic radiation.
 2. Arrangementaccording to claim 1, wherein the means for preventing is adapted toprevent the transmission of line-conducted interference in excess of apredetermined energy level by way of the second lead between the meansfor preventing and the external unit in the direction of the externalunit.
 3. Arrangement according to claim 1, wherein the means forpreventing is arranged adjacent to the outer casing.
 4. Arrangementaccording to claim 2, wherein the means for preventing is of modularconstruction and has means to facilitate fault localization. 5.Arrangement according to claim 1, wherein parts of the control computerthat are located inside the inner casing are designed in accordance withthe personal computer standard, and parts of the control computer thatare located outside the inner casing are designed to comply with astandard for use in an industrial environment.
 6. Arrangement accordingto claim 1, wherein the means for preventing comprises amplitude andfrequency-limiting circuits.
 7. Method of protecting hardware modulesforming part of a control computer against electrical interference, themethod comprising: operatively connecting at least one of the modules toan external unit so that electrical signals can be transmitted betweeneach module and the external unit; passing the signals from the externalunit to each module through interference-preventing means which preventsignal interference that is harmful to the modules from reaching themodules, the interference preventing means including a signal adjustingcircuit; enclosing the control computer in an outer casing to shield thecontrol computer against interference in the form of electromagneticradiation; enclosing the hardware modules in an inner casing to shieldthe modules from electromagnetic interference; designing the parts ofthe control computer that are located inside the inner casing inaccordance with a personal computer standard and designing the parts ofthe control computer that are located outside the inner casing inaccordance with a standard for use in an industrial environment. 8.Method according to claim 7, wherein the interference-preventing meansprevent interference by limiting the signal amplitudes and/or thefrequency interval of the signals.
 9. Method of protecting againstelectrical interference emitted from a control computer that includeshardware modules, the method comprising: operatively connecting at leastone of the modules to an external unit so that electrical signals can betransmitted between each module and the external unit; passing thesignals from each module to the external unit throughinterference-preventing means which prevent signal interference inexcess of a predetermined energy level from passing, the interferencepreventing means including a signal adjusting circuit; enclosing thecontrol computer in an outer casing to shield the control computeragainst interference in the form of electromagnetic radiation emittedfrom the modulus; enclosing the hardware modules in an inner casing toshield the modules from electromagnetic interference: designing theparts of the control computer that are located inside the inner casingin accordance with a personal computer standard and designing the partsof the control computer that are located outside the inner casing inaccordance with a standard for use in an industrial environment. 10.Method according to claim 9, wherein the interference-preventing meansprevent interference by limiting the signal amplitudes and/or thefrequency interval of the signals.